1) Field of the Invention
The present invention relates to a robot structure, notably for minimally invasive surgery.
2) Description of Related Art
In robot assisted minimally invasive surgery instruments, gripping or cutting instruments are provided at the distal end of a robot structure. During a surgical operation, the instruments are located within a patient's body. To be able to operate and move the instruments connected with the robot structure, the robot structure has at least two robot elements interconnected via a hinge or a plurality of joints. Here, a first robot element may be designed as a robot arm and the second, movable robot element comprises at least two gripping elements that are movable against each other, in particular. For moving the movable robot element, the same is connected with a force transmission means that comprises cable controls, for example.
To measure the forces occurring, notably the gripping forces, a sensor element is connected with the robot structure. Presently, such sensor elements are not employed in commercial instruments because of the measurement inaccuracies occurring. Currently, these sensor elements are provided in such robot structures only in instruments for research purposes. It is a general problem that the contact forces and the gripping forces of the movable robot element can be measured only at a very poor resolution, since the forces occurring are superposed by substantially greater driving or motional forces of the force transmission means acting on the movable robot element. Here, gripping forces are those forces generated when gripping an object that is not in contact with the surroundings. Contact forces are generated by the interaction between an instrument or a gripped object and the surroundings. In this instance, the contact forces do not depend on the state of gripping. Contact forces have six degrees of freedom, namely three forces and three moments.
For measuring the gripping force, it is known, for example, to measure the driving or the motional force, respectively. Here, the driving force is a measure of the gripping force, which is why it has no interferences. However, the measurement of the gripping force is relatively inaccurate. Such an instrument is described in J. Rosen, B. Hannaford, M. MacFarlane and M. Sinanan, “Force Controlled and Teleoperated Endoscopic Grasper for minimally Invasive Surgery—Experimental Performance Evaluation”, IEEE Transactions on Biomedical Engineering, 1999; and G. Tholey, A Pillarisetti, W. Green and J. Desai, “Design, Development and Testing of an Automated Laparoscopic Grasper with 3D Force Measurement Capability”, Medical Simulation: International Symposium, ISMS 2004.
Further, it is known for measuring the force occurring, to provide corresponding sensors, such as pressure sensors, in the gripping jaws. This instrument, described in G. Tholey, A Pillarisetti, W. Green and J. Desai, “Design, Development and Testing of an Automated Laparoscopic Grasper with 3D Force Measurement Capability”, Medical Simulation: International Symposium, ISMS 2004, has a drawback, however, in that only little structural space is available for the integration of sensors, since, especially in minimally invasive surgery, the gripping jaws have to be as small as possible.
Further, H. Mayr, I. Nagy, E. Schirmbeck and R. Bauernschmitt, “Upgrading Instruments for Robotic Surgery”, Australian Conference on Robotics & Automation, 2004, describes an instrument wherein Bowden cables are provided to transfer driving forces using a pulling medium. Here, the forces are supported by a rigid sleeve that is flexible but tensionally rigid in the axial direction. In this case, the sleeve serves both to close the force flow of the driving forces, which are rather great forces, and to measure the contact forces that are comparatively weak. The Bowden cables support at least a part of the axial forces to be measured. This causes an impairment of the measurement results. In this instance, the influence of the support depends on the position of the Bowden cables with respect to the sensor. Therefore, the influence on the support may vary and cannot, or only insufficiently, be compensated mathematically.